Cryptography – SSL/TLSwiki.bdnog.org/lib/exe/fetch.php/bdnog7/6.ssl.pdf · protocol which permitted persistent and secure transactions. • In 1997 an Open Source version of Netscape’s

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bdNOG7

19-22 Nov 2017

Dhaka, Bangladesh

Cryptography –SSL/TLS

History

• Secure Sockets Layer was developed by Netscape in 1994 as a protocol which permitted persistent and secure transactions.

• In 1997 an Open Source version of Netscape’s patented version was created, which is now OpenSSL.

• In 1999 the existing protocol was extended by a version now known as Transport Layer Security (TLS).

• By convention, the term "SSL" is used even when technically the TLS protocol is being used.

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SSL/TLS Protocol

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Encrypted SSL/TLS data [HTTP]

Transport (TCP)

Internet

Network Access

Application (HTTP)

TLS/SSL: What it does

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• Confidentiality– Encryption

• Integrity– Keyed hash (HMAC): TLS (authentication!)– Hash (MAC): SSL

• Authentication– certificates

SSL/TLS Operations

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• Client connects to the server– To access a resource

• Public-key cryptography during initial handshake to authenticate and exchange session keys– PKI (X.509 Certificates)

• Symmetric key cryptography to encrypt and hash data– Master secret (shared secret) generated– Separate Encryption and Hashing keys from the master

secret

How SSL/TLS Works – Part 1

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1Sends HelloMessage

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Supported algorithms, random number

Algorithms, random number

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4

Certificate

5 6Encrypted pre-master secret

Sends HelloMessage

AuthenticationServer

SendsCertificates

Generates randomvalue

(pre-master secret &encrypts it

with the server’spublic key)

Decrypts to retrievepre-master secret

7Calculates Keys 7 Calculates Keys

8 8Sends finishedmessage

Sends finishedmessage

9 9Data

SSL Data Transfer Phase

SSL Handshake Phase

CLIENT SERVER

Master Secret (shared)• Encryption key• Hashing key

Encrypted data

PKI

PKI – public key infra

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• Digital (X.509) certificates – associates a public key with an individual or organization– public key of the subject!

Version

Serial Number

Signature Algorithm

Issuer Name

Validity Period

Subject Name

Subject Public Key

Issuer ID

Subject ID

Extensions (CRL)

PKI – Chain of Trust

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• Root CA– Self-signed– Issue and sign ICA’s

certificate

• Intermediate CA– Issue and sign EE

certificate

• End Entity

Root CA

ICA ICA

EE EE EE EE

PKI – Example

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• Client (browser) sends https request to google.com– browsers have trusted CA certificates

stored

• Web server sends back google.com’scertificate– Signed by Google ICA, plus– Google ICA’s certificate signed by root CA

(GeoTrust)

• Verify the certificates up the chain of trust– Once successfully verified, use the public

key

Root CA Cert

Signature (self-signed)

google.comCert

Signature (signed by ICA)

ICA Cert

Signature (signed by root)

How SSL/TLS Works – Part 2

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1Sends HelloMessage

2

Supported algorithms, random number

Algorithms, random number

3

4

Certificate

5 6Encrypted pre-master secret

Sends HelloMessage

AuthenticationServer

SendsCertificates

Generates randomvalue

(pre-master secret &encrypts it

with the server’spublic key)

Decrypts to retrievepre-master secret

7Calculates Keys 7 Calculates Keys

8 8Sends finishedmessage

Sends finishedmessage

9 9Data

SSL Data Transfer Phase

SSL Handshake Phase

CLIENT SERVER

Master Secret (shared)• Encryption key• Hashing key

Encrypted data

Symmetric Encryption

Symmetric Encryption

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• Once the server’s public key is verified up the chain of trust– The client generates a pre-master secret• C-random & S-random

– Sends to the server encrypted with server’s public key

• Both client and server generates the Master Secret– Uses the pre-master secret, C-random, and S-random with

the agreed key exchange cipher (eg: DH)

• Separate Encryption and Hashing keys generated from the Master secret– All future communication hashed and encrypted using the

symmetric keys

Trusted vs Non-trusted Certificate

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Certificate Authority

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Chinese CA WoSign faces revocation after issuing fake certificates of Github, Microsoft and Alibaba

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https://groups.google.com/forum/m/#!topic/mozilla.dev.security.policy/k9PBmyLCi8I/discussion

Introducing Let’s Encrypt

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• An open source CA– Proof your domain to get your digital (TLS/SSL) certificate– https://letsencrypt.org

Let’s Encrypt chain

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• Let’s Encrypt ICA (X3) cross-signed by DST (IdenTrust)– Until ISRG (Internet Security Research Group) is trusted by

everyone

Introducing Let’s Encrypt

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• Browsers and OS– https://community.letsencrypt.org/t/which-browsers-and-

operating-systems-support-lets-encrypt/4394

• Check your browser– https://wiki.apnictraining.net– (signed by Let’s Encrypt)

LABhttp:/ca.apnictraining.net/root-cert

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